The problem is to make it easy for an observer to see that a lightning arrestor or an insulator is carrying a fault current that may be a DC current or a current at mains AC frequency, and that has a magnitude that may, for example, be not less than about 1 amp. In addition, the detection device must be insensitive to current surges due to lightning, which are very large in amplitude (several tens of kiloamps) but extremely short in duration (about a microsecond), i.e. the kind of surge for which lightning arrestors are designed.
French patent application FR-A-2 652 457 discloses a lightning arrestor provided with a visible fault indicator. Use is made of the fact that the lightning arrestor case is inflated under the pressure of gases generated by an internal short circuit within the lightning arrestor. A deformable bracelet surrounding the case is expelled under the effect of such inflation.
Such an indicator suffers from the drawback of operating only with fault currents of several hundreds of amps that give rise to significant amounts of inflation. It is therefore not very sensitive.
French patent application FR-A-2 603 418 discloses an indicator for indicating that a lightning arrestor is in short circuit, which indicator comprises an elastically deformable colored strip clipped against the base of the lightning arrestor by a latch mechanism. The latch is associated with an electrical filter inside the lightning arrestor which, under the effect of a fault current, causes a wire to melt, thereby releasing the latch, and the colored strip is then projected out from the lightning arrestor.
Such an indicator suffers from the drawback of not operating with fault currents of about 1 amp, and of establishing an electrical voltage across its terminals that alters the protection level for which the lightning arrestor is designed in the event of a current surge due to lightning.